The present invention relates to a process for the photo oxidative degradation of a metal salt of an alpha-hydroxy carboxylic acid. More specifically, the invention relates to a process for the photochemical decarboxylation of a metal salt of an alpha-hydroxy carboxylic acid in the absence of a photosensitizing agent to predominantly form the corresponding alcohol.
It has long been known to convert carboxylic acids such as lactic acid, which is an alpha-hydroxy carboxylic acid, to other useful chemicals such as ethanol, methanol, butanol, acetone, acetaldehyde, 2,3-butanediol, acetic acid, propionic acid, alpha-ketoglutaric acid, citric acid, glyoxylic acid, fumaric acid and other polyhydroxy and polycarboxylic compounds. Generally such conversion is accomplished by biological, thermochemical or photochemical means. Biological means, such as fermentation, are time-consuming processes. Thermochemical conversion of carboxylic acids to alcohols in the presence of a catalyst is disclosed in U.S. Pat. No. 3,013,038 to Blair et al. U.S. Pat. No. 3,251,878 to Pasky describes the thermochemical conversion of carboxylic acids to lower molecular weight oxygenated compounds using a metal catalyst. Thermochemical reactions require energy to raise the operating temperature of the process which increases the cost of a commercial operation. For this reason photochemical decarboxylation of alpha-hydroxy carboxylic acids may be preferred.
Euler and Ryd reported in The Decomposition of Lactic Acid and Tannic Acid in Ultraviolet Light, Biochemische Zeitschrift, Vol. 51, pp. 97-103, 1913, that lactic acid undergoes cleavage at 70.degree. C. in the presence of ultraviolet light to form formic acid and acetaldehyde and that these products were in turn rapidly converted to ethanol and carbon dioxide. Euler and Ryd also reported using mineral acids, such as iron and manganese salts, to accelerate the cleavage of lactic acid. Muller disclosed the use of uranyl and iron salts to catalyze the conversion of lactic acid to ethanol. Mueller, R., The Quantum Sensitivity of the Decomposition of Lactic Acid-Uranyl Sulfate, Biochemische Zeitschrift, Vol. 178, pp. 77-78, 1926. Since that time uranyl salts have been popularly used in laboratory studies as photosensitizers to initiate and accelerate chemical reactions driven by light. Thus, uranyl salts enhance both thermochemical and photochemical reactions. However, uranyl salts are expensive and generally not used in commercial processes.
A photosensitizer such as a uranyl salt may be used in conjunction with another catalyst to enhance the performance of the catalyst for the photo-initated conversion of chemicals to desired photoproducts. The combination of photosensitizer and catalyst may produce high yields of a desired product and/or faster reaction rates of conversion when compared to the singular use of the photosensitizer or catalyst. However, processes utilizing both a photosensitizer and a catalyst generally are expensive to operate, as are processes that require a uranyl salt.
In addition, the production of chemicals by way of an alpha-hydroxy carboxylic acid intermediate is generally not a favored reaction path since it is a difficult and lengthy process to obtain yields of essentially pure alpha-hydroxy carboxylic acids, even though yields of the corresponding alcohol may be significant. Processes are known, however, in which the salt of an alpha-hydroxy carboxylic acid may be readily obtained but the photoreactivity of alpha-hydroxy carboxylic acid metal salts may not be similar to that of alpha-hydroxy carboxylic acids.
Thus, it would be a significant contribution to the field of photochemical carboxylic acid degradation to provide a process that produces a significant yield of desired product and/or fast reaction rates from an easily-derived carboxylic acid-based intermediate without requiring the use of both a photosensitizer and a catalyst. Therefore, it is an object of the present invention to provide a process for the photochemical decarboxylation of a metal salt of an alpha-hydroxy carboxylic acid having an improved yield of and/or reaction rate for the formation of the corresponding alcohol.
The above object and other objects of the present invention, together with the advantages thereof, will become apparent from the following specification and appended claims.